Every being in the universe belonged to one of two sexes, or genders (namely male or female), which determined sexual and physiological traits.
Traditionally, members of the male sex acted as a father figure, who provided, cared for, and defended their partner from other males or predators. Sentient and non-sentient species both had male and female sexes, with many biological and childbearing similarities exhibited between the two. Humans generally reproduced by the coupling of a male and female, from which the female would give birth to male or female children. A female could have multiple children over their lifetime, such as Sara Dale Wright. They could also give birth to several at once. Likewise, complications during pregnancy could be disastrous for the mother involved, resulting in permanent disability or death. The children inherited the genetic traits of both their parents.
In most traditional societies, the female sex acted as the caregiver for the child, and took care of the male in his efforts to protect the family. As such, the female would be responsible for providing early education and teaching the child the ways of the world. In developed societies, children could be given to the care of nannies or other supervisors if their parents were unable to spend enough time with their child.
Most societies held certain cultural expectations over the definition of what it meant to be a 'man' and a 'woman.' Typically, such ideas revolved around the idea that men were stronger than women, or at least more capable in the art of warfare, and executing political and intellectual power within the world. As such, women were generally expected to be submissive, subservient, and loyal.
While most individuals exhibited male to female attraction, such as Nate Wright or most of the students attending his school, there also existed a large minority of the international population that sexually identified as preferring male-to-male or female-to-female relationships, or both. Some places were sexually liberal enough for such individuals to make public displays of affection, however most felt awkward, strange, and embarrassed on admitting such facts publicly. Others viewed sexual encounters as non-binding, participating in encounters "just for fun." Many in the world considered such lifestyles "awkward" and "strange," and looked down upon those that practiced it. Despite this, several such individuals existed within the annals of high society. These people were generally referred to as homosexuals, or informally, gays.
Sexes appear in every single Big Nate work. They are also referred to in virtually every novel and many short stories, since describing sexes is describing people in general.